Isocyanates are widely used as production raw materials of products, such as polyurethane foams, paints and adhesives. The main industrial production process of isocyanates includes reacting amine compounds with phosgene (phosgene method), and nearly the entire amount of isocyanates produced throughout the world are produced according to the phosgene method. However, the phosgene method has numerous problems.
Firstly, this method requires the use of a large amount of phosgene as raw material. Phosgene is extremely toxic and requires special handling precautions to prevent exposure of handlers thereof, and also requires special apparatuses to detoxify waste.
Secondly, since highly corrosive hydrogen chloride is produced in large amounts as a byproduct of the phosgene method, in addition to requiring a process for detoxifying the hydrogen chloride, in many cases hydrolytic chlorine is contained in the isocyanates produced, which may have a detrimental effect on the weather resistance and heat resistance of polyurethane products in the case of using isocyanates produced using the phosgene method.
On the basis of this background, a process for producing isocyanates has been sought that does not use phosgene. One example of a method for producing isocyanate compounds without using phosgene that has been proposed involves thermal decomposition of carbamic acid esters. Isocyanates and hydroxy compounds have long been known to be obtained by thermal decomposition of carbamic acid esters (see, for example, Non-Patent Document 1). The basic reaction is illustrated by the following formula:R(NHCOOR′)a→R(NCO)a+aR′OH  (1)(wherein R represents an organic residue having a valence of a, R′ represents a monovalent organic residue, and a represents an integer of 1 or more).
On the other hand, thermal decomposition of carbamic acid esters is susceptible to the simultaneous occurrence of various irreversible side reactions such as thermal denaturation reactions undesirable for carbamic acid esters or condensation of isocyanates formed by the thermal decomposition. Examples of these side reactions include a reaction in which urea bonds are formed as represented by the following formula (2), a reaction in which carbodiimides are formed as represented by the following formula (3), and a reaction in which isocyanurates are formed as represented by the following formula (4) (see, Non-Patent Document 1 and Non-Patent Document 2).

In addition to these side reactions leading to a decrease in yield and selectivity of the target isocyanate, in the production of polyisocyanates in particular, these reactions may make long-term operation difficult as a result of, for example, causing the precipitation of polymeric solids that clog the reaction vessel.
Various methods have been proposed for producing isocyanates that do not contain phosgene.
According to the description of Patent Document 1, an aliphatic diurethane and/or alicyclic diurethane and/or aliphatic polyurethane and/or alicyclic polyurethane are obtained by reacting an aliphatic primary diamine and/or alicyclic primary diamine and/or aliphatic primary polyamine and/or alicyclic primary polyamine in the presence of an O-alkylcarbamate and alcohol and in the presence or absence of a catalyst at 160 to 300° C. at a ratio of amine NH2 group:carbamate:alcohol of 1:0.8 to 10.0:0.25 to 50, and removing the ammonia formed as necessary. The resulting diurethane and/or polyurethane can be converted to the corresponding diisocyanate and/or highly functional polyisocyanate as necessary. The detailed reaction conditions for the thermal decomposition are not described in this patent publication.
According to the description of Patent Document 2, an aromatic diisocyanate and/or polyisocyanate are produced by going through the following two steps. In the first step, an aromatic primary amine and/or an aromatic primary polyamine are reacted with an O-alkylcarbamate in the presence or absence of a catalyst and in the presence or absence of urea and alcohol to form an aryl diurethane and/or aryl polyurethane followed by removal of the ammonia formed as necessary. In the second step, an aromatic isocyanate and/or aromatic polyisocyanate are obtained by thermal decomposition of the aryl diurethane and/or aryl polyurethane.
Other publications contain descriptions relating to partial substitution of urea and/or diamine by a carbonyl-containing compound such as N-substituted carbamate and/or dialkylcarbamate, or mono-substituted urea, di-substituted urea, mono-substituted polyurea or di-substituted polyurea (see Patent Document 3, Patent Document 4, Patent Document 5, Patent Document 6 and Patent Document 7). Patent Document 8 describes a method for producing aliphatic O-aryl urethanes by reacting (cyclic) aliphatic polyamines with urea and aromatic hydroxy compounds.
There are several known methods for forming corresponding isocyanates and alcohols by thermal decomposition of (cyclic) aliphatic, and particularly aromatic, monourethanes and diurethanes, including methods carried out in the vapor phase at a high temperature, and methods carried out in a liquid phase under comparatively low temperature conditions. In these methods, however, the reaction mixture gives rise to the side reactions described above, thereby causing, for example, the formation of sediment, polymeric substances and obstructions in the reaction vessel and recovery apparatus, or the formation of substances that adhere to the sidewalls of the reaction vessel, thereby resulting in poor economic efficiency in the case of producing isocyanates over a long period of time.
Thus, the use of a chemical method, such as the use of a special catalyst (see Patent document 9 and Patent Document 10) or a catalyst combined with an inert solvent (see Patent Document 11), has been disclosed to improve the yield in the thermal decomposition of urethane.
For example, Patent document 12 describes a process for producing hexamethylene diisocyanate including thermal decomposition of hexamethylene diethyl urethane in the presence of dibenzyl toluene used as a solvent and in the presence of a catalyst mixture composed of methyl toluene sulfonate and diphenyl tin dichloride. However, since there are no detailed descriptions of production or isolation of the starting components or purification and arbitrary recovery of the solvent and catalyst mixture, the economic efficiency of this process could not be evaluated.
According to the method described in Patent Document 13, urethane can be easily broken down into isocyanate and alcohol in a carbon-containing fluidized bed without using a catalyst. In addition, according to the description of Patent document 14, hexamethylene dialkyl urethane can be decomposed in the gaseous phase at a temperature in excess of 300° C. in the presence or absence of a gas permeable packaging material made of, for example, carbon, copper, brass, steel, zinc, aluminum, titanium, chromium, cobalt or quartz to form hexamethylene diisocyanate.
In addition, according to the description of Patent Document 14, this method is carried out in the presence of a hydrogen halide and/or hydrogen halide donor. However, this method is unable to achieve a yield of hexamethylene diisocyanate of 90% or more. This is because the decomposition products are partially recombined resulting in the formation of urethane bonds. Thus, the hexamethylene is required to be additionally purified by distillation, which frequently increases yield loss.
Moreover, according to the description of Patent Document 15, a monocarbonate is disclosed to be able to be decomposed with favorable yield and in the absence of a solvent and in the presence or absence of a catalyst and/or stabilizer at a comparatively low temperature and advantageously under reduced pressure. The decomposition products (monoisocyanate and alcohol) are removed from a boiling reaction mixture by distillation and separately captured by separative condensation. A method for removing byproducts formed during thermal decomposition including partially removing the reaction mixture outside the system is described in a generic form. Thus, although byproducts can be removed from the bottom of the reaction vessel, problems remain with respect to the case of adherence to the sidewalls of the reaction vessel as previously described, and problems with respect to long-term operation remain unsolved. In addition, there is no description regarding industrial utilization of the removed reaction mixture (containing large amounts of useful components).
According to the description of Patent Document 16, thermal decomposition of aliphatic, alicyclic or aromatic polycarbamates is carried out in the presence of an inert solvent at 150 to 350° C. and 0.001 to 20 bar, and in the presence or absence of a catalyst, and an assistant in the form of hydrogen chloride, organic acid chloride, alkylation agent or organic tin chloride. Byproducts formed, can be continuously removed from the reaction vessel together with the reaction solution, for example, and a corresponding amount of fresh solvent or recovered solvent can is added simultaneously. A shortcoming of this method is, for example, a reduction in the space-time yield of polyisocyanates as a result of using a refluxing solvent, while also requiring considerable energy, including that for recovery of the solvent. Moreover, the assistants used are volatile under the reaction conditions, resulting in the potential for contamination of the decomposition products. In addition, the amount of residue is large based on the formed polyisocyanate, thus leaving room for doubt regarding economic efficiency and the reliability of industrial methods.
According to the description of Patent Document 17, a method is described for continuous thermal decomposition of a carbamate such as the cyclic diurethane, 5-(ethoxycarbonylamino)-1-(ethoxycarbonylaminomethyl)-1,3,3-trimethylcyclohexane, supplied along the inner surface of a tubular reaction vessel in a liquid form in the presence of a high boiling point solvent. This method has the shortcomings of low yield during production of (cyclic) aliphatic diisocyanates and low selectivity. In addition, there is no description of a continuous method accompanying recovery of recombined or partially decomposed carbamates, while there is also no mention made of post-treatment of solvent contained in the byproducts and catalyst.
According to Patent Document 18, a circulation process is disclosed for producing (cyclic) aliphatic diisocyanates by converting corresponding diamines to diurethane followed by thermal decomposition of this urethane. This process minimizes reductions in yield by recirculating the product of the urethane decomposition step following reaction with alcohol to a urethanation step. Byproducts unable to be recirculated are removed by separation by distillation of the mixture of urethanation products, and in this case, a valueless residue is formed in the form of bottom products, while all comparatively low boiling point components, including diurethane, are removed from the top of the column. However, this process has the shortcoming of using a large amount of energy. This is because all of the diurethane must be evaporated in the presence of catalyst, and this diurethane must be evaporated at a temperature level within the range of the decomposition temperature of urethane. Moreover, isocyanate groups formed in useful products react with residue urethane groups, frequently resulting in the formation of comparatively high molecular weight byproducts that lower yield.
In addition, according to the description of Patent Document 19, a method is disclosed for partially removing valueless byproducts outside the system during thermal decomposition of polyurethane. A shortcoming of this method is that isocyanate yield decreases due to polyurethane ending up being contained in byproducts partially removed outside the system. In addition, heating of byproducts remaining in the reaction vessel without being removed outside the system results in the formation of polymeric compounds, and the adherence of these compounds to the reaction vessel makes continuous operation over a long period of time difficult.    Patent Document 1: U.S. Pat. No. 4,497,963;    Patent Document 2: U.S. Pat. No. 4,290,970;    Patent Document 3: U.S. Pat. No. 4,388,238;    Patent Document 4: U.S. Pat. No. 4,430,505;    Patent Document 5: U.S. Pat. No. 4,480,110;    Patent Document 6: U.S. Pat. No. 4,596,678;    Patent Document 7: U.S. Pat. No. 4,596,679;    Patent Document 8: European Patent Application Laid-open No. 0320235;    Patent document 9: U.S. Pat. No. 2,692,275;    Patent Document 10: U.S. Pat. No. 3,734,941;    Patent Document 11: U.S. Pat. No. 4,081,472;    Patent document 12: U.S. Pat. No. 4,388,426;    Patent Document 13: U.S. Pat. No. 4,482,499;    Patent document 14: U.S. Pat. No. 4,613,466;    Patent Document 15: U.S. Pat. No. 4,386,033;    Patent Document 16: U.S. Pat. No. 4,388,246;    Patent Document 17: U.S. Pat. No. 4,692,250;    Patent Document 18: European Patent Application No. 0355443;    Patent Document 19: Japanese Patent No. 3382289;    Non-Patent Document 1: Berchte der Deutechen Chemischen Gesellschaft, Vol. 3, p. 653, 1870; and    Non-Patent Document 2: Journal of American Chemical Society, Vol. 81, p. 2138, 1959.